Testing forages and interpreting the results
Forage testing is a tool to determine current nutritive values as well as non-nutritive values in forages that can be used for marketing hay, formulating rations, and determining potential toxicities or other problems (i.e., prussic acid, nitrate, alkaloid, etc.).
How to Sample for Forage Testing
The most important thing in forage sampling is to have a representative sample. The proper method of sampling depends on the type of forage.
- Hay: For rectangular square bales, forage sampling should be done by coring in from the ends. For large round bales, forage samples should be taken from the sides not the front or back, to get a cross section of the rolled hay. When sampling hay from bales, it is important to use a core sampler rather than using hands to get a representative forage sample. Take a separate sample from each field and cutting; otherwise, forage quality analysis can give you misleading information. Take at least 20 core samples from each hay lot, composite the samples, mix them thoroughly, and take a sub-sample for analysis. Put the sub-sample into a clean, airtight plastic bag with a label including your name, address, forage type, stage of maturity, and date harvested.
- Haylage and Silage at Harvest: It is important to take samples as the silage is placed in the silo. Take three to five handfuls of haylage or silage from every third load or more and place them in a plastic bag with all samples from the same field. It’s important to store the samples in either refrigerator or freezer immediately until it is shipped to the lab for a forage quality analysis. Composite the samples, mix them well to obtain a representative sample and take a sub-sample (about 2 pounds or one pint) for analysis.
- Haylage and Silage from Storage: In addition to sampling haylage and silage at harvest, it is also important to take samples from storage. Silage quality can vary with silage-making techniques and management skills. Collect one- to two-pound samples from the silo as it is discharged from the silage unloader. In upright silos, it is important to wait before sampling until two to three feet of silage has been removed. In any type of silo, do not collect samples from spoiled material on top of the silo; otherwise this sample analysis can give you misleading information. Collect samples from the morning and evening feedings over a two-day period. Mix the samples well, place in a clean plastic bag, seal, and store immediately in a cold place such as refrigerator or freezer until samples are shipped to the lab.
Common Terms and Interpretation of Forage Quality Analysis Results
- Moisture is the water present in the forage and is expressed as percent.
- Dry matter (DM) is the percentage of the forage that is not water. 100 minus moisture content gives dry matter percentage (i.e., if moisture is 65%, then dry matter percentage is 35%, 100 - 65% = 35%)
- Crude protein is the amount of nitrogen in the forage. It is the sum of true protein and non-protein nitrogen. True protein such as microbial protein is utilized in rumen as the food for rumen microbes. Non-protein nitrogen, such as urea, is utilized in the small intestine. Since protein has about 16% nitrogen, crude protein can be obtained by multiplying percent nitrogen by 6.25 (i.e., if nitrogen concentration is 3%, the crude protein would be 3% N x 6.25 = 18.75%)
- Acid detergent fiber (ADF) is the sum of hemicellulose, cellulose, and lignin. ADF is used as a predictor of digestibility and energy value, and is inversely related to digestibility (i.e., as the ADF percentage increases, then digestibility and energy value decrease).
- Neutral detergent fiber (NDF) is the sum of hemicellulose and lignin. NDF is a predictor of feed intake potential or gut fill in rumen, and is inversely related to feed intake (i.e., if the NDF is low, then feed intake can be high). In general, forage legumes tend to have lower NDF values than grasses although it depends on the stage of maturity at harvesting.
- Total digestible nutrients (TDN): TDN is directly related to digestible energy and is the sum of digestible fiber, starch, sugars, protein, and fat in the forage. TDN is useful for beef cow rations that are primarily forage.
- Net energy maintenance (NEM) and lactation (NEL): Net energy is the energy concentration in a feed. It can be measured by laborious animal trials or can be predicted using either ADF or NDF. Older, mature forages generally have higher fiber and less energy than younger, succulent forages. Thus older, mature forages have lower net energy values than younger forage plants. Most dairy producers generally use NEL to balance rations for lactating cows while some beef producers use NEM.
- Relative forage value (RFV): RFV is an index used to rank forages based on ADF and NDF values. No unit value is used for RFV. It measures overall feed value of forage and it is used in hay markets, in particular alfalfa.
- Relative forage quality (RFQ): RFQ is newer index than RFV and is based on intake and TDN. One of the big differences in RFQ compared to RFV is that RFQ uses fiber digestibility to estimate intake as well as total digestible nutrients (TDN). This RFQ value can be better and a more accurate predictor than RFV for both warm-season and cool-season forages.
Doo-Hong Min, Southwest Area Crops and Soils Specialist
John Holman, Cropping Systems Agronomist, Southwest Research-Extension Center